Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jul. 01, 2017	Dec. 31, 2016
Cost
Due in one year or less, Cost	$ 293,156
Due after one year through ten years, Cost	179,050
Due after ten years, Cost	96,260
Total Cost	568,466
Fair Value
Due in one year or less, Fair Value	293,132
Due after one year through ten years, Fair Value	179,051
Due after ten years, Fair Value	95,639
Total Fair Value	567,822
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	195,227	$ 101,480
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,379	5,196
Total fair value of available-for-sale securities, continuous loss position	200,606	106,676
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(221)	(288)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(621)	(804)
Available-for-sale securities, total gross unrealized losses	(842)	(1,092)
Other than temporary impairment losses
Other-than-temporary impairment losses	0
Government debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	110,741	79,743
Total fair value of available-for-sale securities, continuous loss position	110,741	79,743
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(97)	(156)
Available-for-sale securities, total gross unrealized losses	(97)	(156)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	84,486	21,737
Total fair value of available-for-sale securities, continuous loss position	84,486	21,737
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(124)	(132)
Available-for-sale securities, total gross unrealized losses	(124)	(132)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,379	5,196
Total fair value of available-for-sale securities, continuous loss position	5,379	5,196
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(621)	(804)
Available-for-sale securities, total gross unrealized losses	$ (621)	$ (804)